Densification is a sustainable urbanization strategy for reducing per capita environmental impacts facilitating the achievement of sustainable cities as outlined in the sustainable development goals. We have developed a methodology for assessing sustainable densification potentials at a national scale based on urban structure units and perform an analysis for residential Switzerland. In contrast to single-building assessments, our methodology allows a differentiated spatial identification and evaluation of entire neighbourhoods. We focus on urban post-war neighbourhoods, which are of key importance for realising sustainable densification. Depending on the chosen methodological assumptions, between 1.1 - 1.6 million people are currently living in post-war neighbourhoods in Switzerland. We further classify post-war neighbourhoods into different geographical centrality classes to evaluate the suitability for densification from a sustainability point of view. We have calculated a first estimate of densification potentials of postwar neighbourhoods, i.e. the number of additional inhabitant which could be accommodated, across a range of density values. We estimate, that densifying post-war neighbourhoods to a minimum density value of 200 inhabitants per hectare building zone would enable the accommodation of an additional 4 - 10% of today's Swiss population. More than half of this potential is located in central or very central urbanised geographical locations and thus most interesting from a sustainability point of view.